Pneumatic rubber tires conventionally have visually exposed external components of one or more rubber compositions which, when exposed to weathering atmospheric conditions and dynamic use, tend to discolor to various degrees and to provide an external tire appearance which is not typically uniform in nature. For example, a black colored tire sidewall may tend to become brown in color as a result of migration of antidegradants to the surface of the rubber composition, together with exposure to atmospheric ozone, ultraviolet light and weathering in general. Such phenomena are well known to those having skill in such art.
When a tire is initially manufactured, the associated tire molding surface conventionally provides an attractive appearance for the surface of the entire tire.
As the tire naturally ages, or is used, under atmospheric conditions, the appearance of a tire sidewall, for example, conventionally becomes somewhat duller in nature due to atmospheric elements such as exposure to ozone and ultraviolet light. Also, the visually appearing tire sidewall conventionally often becomes somewhat discolored, due to migration of, for example, rubber processing oil(s), particularly aromatic rubber processing oils, which may be contained within the rubber composition of the tire component, as well as migration of paraffinic or microcrystalline waxes, if used, and to some extent staining antidegradants, if used, to the exposed surface of the rubber composition of a visually exposed component of the tire.
Such tire components having visually exposed surfaces may be, for example, tire sidewalls, tire chafers, and tire treads, or individual portions thereof.
According to this invention, a method is provided for masking, retarding, and possibly essentially eliminating for an extended period of time, such dulling and/or discoloring (usually browning) of the exposed surface of a tire component due to such migration of various ingredients and/or exposure to atmospheric conditions.
It is to be appreciated that this invention is intended to go beyond a simple elimination of one or more offending ingredients, which in and of itself however might result in a degree of reduction of aged degradation of the visible surface of the rubber component, or perhaps preventing the exposure of the tire to atmospheric conditions which in and of itself would be difficult to accomplish.
In practice, this invention relies upon an inclusion of one or more selective additives to one or more visually exposed components of a tire (e.g. sidewall) which have one or more visibly exposed surfaces which have a tendency to migrate to a visible surface of the tire component and, also, to thereby form a film, or barrier, preferably a relatively glossy film, on the visible surface of the tire component.
Such additive(s) are desirably characterized by being relatively incompatible with and by not being easily sulfur curable with elastomers, at least diene-based elastomers, as well as being of a relatively low molecular weight and of a relatively low viscosity.
Such additives are desirably characterized by being both non-polar and polar in nature along the chemical molecule. The non-polar portion of the molecule should exhibit limited compatibility with the diene based elastomers, whereas the polar portion will be somewhat incompatible and tend to migrate to the cured rubber surface.
In practice, it is considered herein that such incompatibility of the polar function of the additive(s), in combination of its relatively low molecular weight, relatively low viscosity, and/or relatively low melting point, enhances its tendency to migrate to the visual surface of the tire component and tend to create a continuous film on the surface which can mask the aforesaid discoloration and/or dullness of the surface caused by migration of other ingredients within the rubber composition to its visible surface and exposure to atmospheric elements.
Accordingly, one aspect of the philosophy of this invention is to create and/or maintain an aesthetically appealing visible surface of a respective tire component over an extended period of time by an inclusion of such selective ingredient(s) in the respective rubber composition of selective tire component(s) which has a tendency to migrate to the visible surface of the tire component to substantially mask the effects of migration of rubber processing oils, staining antidegradants or paraffinic waxes and various microcrystalline waxes and effects of atmospheric aging.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.